virtio: fix whitespace in virtio_config_ops

The finalize_features documentation uses a tab between words.
Use space instead.

Fixes: d16c0cd2 ("docs: driver-api: virtio: virtio on Linux")
Message-Id: <39d7685c82848dc6a876d175e33a1407f6ab3fc1.1763026134.git.mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>